Susceptor for vapor-phase growth apparatus

ABSTRACT

It was an objective of the present invention to provide a susceptor which can prevent a increasing phenomenon of the dopant concentration of the epitaxial layer at the peripheral portion of the wafer. By providing a through-hole  7  passing through to a rear side at the outer peripheral side of the wafer inside the wafer pocket  6 , a down flow of a reacting source gas from the upper surface of the susceptor  5  is formed, so that the unwanted flow of the dopant species being exhausted at the rear surface onto the wafer surface can be avoided. As a result, a raise in the dopant concentration at the outer peripheral portion of the epitaxial layer  9  can be controlled.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improvement of a susceptor which isemployed to a vapor-phase growth apparatus to grow an epitaxial filmonto the semiconductor wafer. More specifically, the present inventionrelates directly to a vapor-phase growth apparatus in which athrough-hole portion extending to a rear side of the susceptor isprovided at the most outer peripheral portion inside the wafer pocket inorder to mount the wafer, and a raise in dopant concentration at theouter periphery of the grown epitaxial film can be controlled.

2. Description of the Prior Art

As to a vapor-phase growth apparatus in order to grow an epitaxial filmonto the semiconductor wafer, there have been several conventional typesof apparatus available; they may include (1) a vertical-type vapor-phasegrowth apparatus in which the susceptor being placed on a circular discis heated from its bottom side, and (2) a single wafer type vapor-phasegrowth apparatus with which a good quality epitaxial film can befabricated.

For example, inside the rectangular chamber being fabricated of quartzin said single wafer type vapor-phase growth apparatus, semiconductorwafer is mounted on the disc-shaped susceptor which is a graphite beingcoated with SiC. The semiconductor wafer is heated using a heater whichis provided outside of the chamber in order to react with various typesof reacting source gases passing through the chamber, resulting ingrowing the epitaxial film on the semiconductor wafer.

As seen in FIG. 3, said susceptor 5 is composed of a high puritygraphite which is coated mainly by SiC. On the surface, a groove calledas a wafer pocket 6 is formed in order to accomodate the silicon wafer.The size of said wafer pocket 6 is slightly larger than the dimension ofthe wafer 8 and the depth is about 1 mm. After placing the wafer 8inside the wafer pocket 6, the susceptor is held in the reacting sourcegaseous flow at a predetermined temperature to generate the siliconepitaxial film layer 9 on the wafer surface.

Moreover, several improvements have been proposed in order to minimizethe surface contact between the inner surface of the pocket and thewafer rear surface. These proposed improvements may include (1) astructure to contact-hold the wafer to a plurality of convex portions byforming a mesh-shaped shallow fine groove so-called a roulette, (2)making a tapered surface in order to confine the contact of the wafer atits outer periphery, or (3) using much coarser surface roughness ofcoated SiC surface than the surface roughness of the wafer.

As to a reacting source gas, a dopant source gas such as diborane (Ptype) or phosphine (N type) is added to a chloro-silane gas which ishydrogen diluted. Hence the silicon epitaxy as well as a bi-product ofHCl are produced on the wafer surface through a heat CVD (chemical vapordeposition) reaction. As a result, although the silicon epitaxial growthon the wafer surface can proceed, the rear surface of said wafer is alsoexposed to diffusion reaction gas to create a Si—H—Cl atmosphere, whichmight furthermore lead to a precipitation/etching reaction in amicroscopic scale.

For instance when the epitaxial growth having lower concentration thanthe dopant concentration of the wafer is conducted such as an epitaxialgrowth of P type film (specific resistance is 1 Ωcm) against the waferwith the dopant concentration P ++ type (specific resistance is 5 mΩcm),the dopant concentration in the epitaxial layer tends to increase at theouter peripheral portion of the wafer, as demonstrated in FIG. 4 whichshows a change in dopant concentration from the center of the wafer andas a function of a distance from the center to the most outer periphery.

The above phenomenon might be due to the fact that the dopant species ofthe wafer 8 might be exhausted in Si—H—Cl atmosphere at the rear surfaceof wafer 8, and the exhausted dopant species might migrate to the frontsurface through the gaseous diffusion flow 11, resulting in increasingthe dopant gaseous concentration locally. As a result, a particularregion of the epitaxial layer where the dopant concentration is out ofthe range defined by the specification, leading to a poor productionefficiency of the device.

SUMMARY OF THE INVENTION

Objective of the Invention

All of the forgoing have resulted in a requirement for improvement ofthe apparatus of the present invention in which it is an objective ofthe present invention to provide a susceptor which can prevent theincreasing phenomenon of the dopant concentration in the epitaxial layerat its peripheral portion, as it would be obvious when the epitaxialgrowth proceeds at lower concentration than the dopant concentration ofthe wafer. It is, accordingly, another objective to provide a susceptorfor the vapor-phase growth apparatus which can avoid the unwanted flowof the dopant species being exhausted at the rear side to the wafersurface.

Disclosure of the Invention

The present inventors found that, in a suscpetor for the vapor-phasegrowth apparatus, the aforementioned localized nonuniform distributionof the dopant concentration can be minimized by forming a vapor flow inorder to prevent the unwanted flow of the dopant species being exhaustedat the rear side to the wafer surface. After investigating variousdesigns for the susceptor to achieve said objectives, the followingdesign was evaluated to perform the best efficiency. By providing athrough-hole passing through to the rear side at the outer peripheralportion of the wafer inside the wafer pocket, the down-flow from theupper surface of the susceptor is generated, so that the unwanted flowof the dopant species being exhausted toward the wafer surface can beprevented. As a result, the raise in the dopant concentration can becontrolled at the outer peripheral portion of the epitaxial layer.

Namely, according to the present invention, a susceptor can be providedwhich is characterized by providing a through-hole passing through tothe rear side at the most outer peripheral portion inside the waferpocket which is a concave portion for mounting the wafer.

According to the present invention, the localized raise of the dopantconcentration at the most outer peripheral portion of the grownepitaxial layer can be prevented by providing a through-hole passingthrough to the rear side of the susceptor at the outer periphery insidethe wafer pocket which is used for mounting the wafer. Specifically, theraise in the dopant concentration in the epitaxial layer can be avoidedwhen the epitaxial growth with a lower concentration than the dopantconcentration of the wafer is progressing.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and many other objectives, features and advantages of thepresent invention will be more fully understood from the ensuingdetailed description of the preferred embodiment of the invention, whichdescription should be read in conjunction with the accompanyingdrawings.

FIG. 1a is a view explaining a vapor-phase growth apparatus employing asusceptor, according to the present invention.

FIG. 1b is another view describing the cross-sectional view of thesusceptor along its radius direction.

FIGS. 2a and 2b are upper half views of the susceptor, according to thepresent invention.

FIG. 3 shows a cross-sectional view of the conventional type ofsusceptor along its radius direction.

FIG. 4 shows changes in a dopant concentration as a function of adistance from the center of the wafer along its radius direction.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A vapor-phase growth apparatus seen in FIG. 1a has a rectangular chamber1 made of quartz, in which the semiconductor wafer 8 is mounted on thecircular disc-shaped susceptor 5 which is a graphite substrate beingcoated with SiC. The semiconductor wafer 8 is heated by a heater (notshown) which is placed outside of the chamber 1 to react with thereacting source gas flowing horizontally inside the chamber 1 in orderto generate the epitaxial film 9 on the semiconductor wafer surface 8.

The susceptor 5 of the present invention has an arc-shaped groove-typethrough-hole portion 7 at the most outer peripheral portion inside thewafer pocket 6 for mounting the semiconductor wafer 8. The inner plainportion of the wafer pocket is a coated SiC layer.

In FIG. 1a, although the reacting source gas 10 is introducedhorizontally from the gas inlet opening 2 of the chamber 1 to the gasexhaust opening 3, the source gas is supplied and heated particularly tothe wafer surface 8 and the bi-product gas is generated during theepitaxial reaction, so that the gas can be subjected to the volumetricexpansion. By providing a through-hole portion 7 at the outer peripheralportion inside the wafer pocket of the susceptor 5, a localized gas flowis generated from the wafer surface 8, and the boron species beingexhausted from the rear surface of the wafer 8 can be exhausted withoutthe unwanted flow-back to the front surface of the wafer 8.

Furthermore, by blowing the source gas directly toward the front surfaceof the wafer 8, the gas flow 12 passing from the front surface to therear surface of the wafer 8 through the through-hole can be promoted, sothat the growth efficiency can be enhanced. Moreover, by providing anexhaust opening 4 at the rear side of the susceptor 5 in the chamber 1,the gas flow passing from the front surface to the rear surface of thewafer 8 through the through-hole portion 7 can also be promoted,resulting in that the epitaxial growth efficiency can be improved.

In the above, although the single wafer type vapor-phase growthapparatus has been described, the type of the apparatus to which thepresent invention is applicable includes any prior art types including avertical-type vapor-phase growth apparatus or a barrel-type vapor-phasegrowth apparatus. With any one of these types of growth apparatus,according to the present invention, the reacting source gas can flow inparallel to the wafer surface being placed in the susceptor.

The through-hole portion of the susceptor in this invention can bevarious types including an arc-shaped groove-type through-hole portionas mentioned previously, an ovalshaped through-hole portion, or aplurality of small size of holes. Moreover, with the single wafer typeof vapor-phase growth apparatus supporting wafers at the central portionof the susceptor, if through-holes as many as possible can be provided,the exhausting efficiency of the boron species from the rear surface ofthe wafer could be enhanced in such a way that said many through-holesare designed and fabricated with relatively large connecting area left,so that the area can be strong enough to withstand the weight of theouter peripheral portion area by the wafer pocket. Similarly, with anyother susceptor types than the single wafer type, it is recommended toprovide through-holes as many as possible if there is an enoughconnecting portion left to withstand the structural strength underconsidering the wafer weight. Furthermore, it is preferable to definethe diameter (or width) of the through-hole along the wafer direction tobe, at most, equal to the wafer's outer periphery under taking theheating effect into account.

EMBODIMENTS

Using the horizontal single wafer type vapor-phase growth apparatus witha lamp-heating method as seen in FIG. 1, an epitaxial film with the filmthickness of about 10 pm was formed at a reaction temperature of 1,150°C. onto the P++ type (100) plane silicon semiconductor base plate (with200 mm diameter) having the specific resistance of 5 mΩcm using SiHCl₃diluted with hydrogen as the silicon supplying source gas. Two testswere conducted; one was with susceptor having the through-hole of thepresent invention and the other was with the conventional type ofsusceptor without any through-holes as seen in FIG. 3.

As seen in FIGS. 2a and 2b, the suscpetor according to the presentinvention has an arcshaped groove-type through-hole portion 7 at themost outer peripheral portion of the wafer pocket 6. In FIG. 2a, fourlocations are installed with the through-hole portions 7 leaving theconnecting area of 75 mm on its peripheral portion. On the other hand,in FIG. 2b, four locations are provided with through-hole portions 7leaving 5 mm connecting area on its peripheral portion. The lengthfractions of said each through-hole portion per the total peripherallength was approximately 50% and 90%, respectively.

The raise in the dopant concentration of the outer peripheral portion ofthe grown epitaxial film was listed in Table 1 and presented in FIG. 4.By comparing with the conventional type as seen in FIG. 3, it was foundthat the longer the through-hole portion in both FIG. 2a and FIG. 2b,the lesser the dopant concentration increase. Table I. Comparison ofdopant concentration at center and 3 mm from the edge for conventionaltype susceptor and the susceptor of the present invention.

TABLE I 3 mm from center location the edge increment (× 10¹⁶ atoms/ (×10¹⁶ atoms/ (× 10¹⁵ atoms/ susceptor cm³) cm³) cm³) conventional type1.00 1.20 2.0 (FIG. 3) Example 1 1.00 1.10 1.0 (FIG. 2a) Example 2 1.001.05 0.5 (FIG. 2b)

While the invention has been explained with reference to the structuredisclosed herein, it is not confined to the details as set forth, andthis application is intended to cover modifications and changes as maycome within the scope of the following claims.

1. A susceptor for a vapor-phase growth apparatus comprising at leastone through hole passing through a rear side at an outer peripheralportion of a wafer pocket which is formed as a concave shape in order tomount said wafer.
 2. The susceptor of claim 1, wherein the at least onethrough hole is shaped as a groove and a plurality thereof is providedon a periphery.
 3. A method for growing an epitaxial layer having alower dopant concentration than the dopant concentration of a wafer, themethod comprising: positioning the wafer in relation to a susceptor, thesusceptor having a through hole; providing a reacting source gas to thewafer; and forming a vapor flow through said through-hole to providevapor flowing from an upper surface of the susceptor to a lower surfaceof the susceptor and thereby preventing flow of a dopant species from arear side of the wafer to a surface of the wafer.
 4. A method inaccordance with claim 3 wherein the susceptor further comprises a waferpocket having a concave surface for mounting the wafer and wherein thethrough-hole passes-from the concave surface to the rear side of thesusceptor.
 5. The method according to claim 3 further comprisingproviding the through-hole on an outer peripheral side of the wafer. 6.The method according to claim 4 further comprising providing thethrough-hole on an outer peripheral side of the wafer.